(a) Field of the Invention
The present invention relates to a method for canceling a partially overlapped single-carrier crosstalk signal and an apparatus using the same. More particularly, the present invention relates to an apparatus for cancellation of a partially overlapped crosstalk signal generated due to overlapping of a transmit frequency bandwidth of a transmitting apparatus and a receive frequency bandwidth of a receiving apparatus in a mobile communication environment for data transmission, and a method thereof.
(b) Description of the Related Art
The development of communication and network technologies has provided various types of multimedia services such as e-mail, chatting, messengers, games, movies, and music. Particularly, many users require multimedia services through a wireless Internet service since the users can use the wireless Internet service anytime and anywhere.
An increasing demand for the multimedia services requires a higher data transmission speed for multimedia service contents transmission, and the higher data transmission speed can be achieved by increasing spectral efficiency, a data symbol transmission rate, or a signal bandwidth.
However, increase of the symbol rate may not always be feasible in practice due to the limited available bandwidth and complexity in implementation of a mobile communication system. For this reason, the demand for the high speed transmission cannot be solved by simply increasing the data transmission speed.
In a digital subscriber line (DSL) environment, different frequency bands are used for a transmitter (TX) and a receiver (Rx) for duplex operation. For example, the increase of the symbol rate can be achieved without increasing the overall bandwidth by making the Tx and Rx spectra overlap each other in the frequency domain. In this case, a crosstalk signal due to imperfect wire shielding must be suppressed.
Conventionally, an inband data-driven crosstalk canceller (IDXC) scheme is used to cancel the crosstalk signal.
The IDXC scheme processes the crosstalk signal at a Nyquist rate, which can be implemented by using a polyphase structure, and therefore, the IDXC scheme provides reliable performance while being robust against timing and frequency offset. Herein, the Nyquist rate can be used to perform signal transmission without causing inter-symbol interference.
However, when an overlapped bandwidth of the crosstalk signal is smaller than a bandwidth of a Tx signal, a conventional Nyquist rate IDXC uses a sampling frequency much higher than the bandwidth of the overlapped crosstalk, requiring large computational complexity.
FIG. 1 shows that the spectrum of a single-carrier crosstalk signal is formed by the partially overlapped spectra of the Tx and Rx signals.
As shown in FIG. 1, assuming that the Tx and Rx spectra are partially overlapped, a spectrum fe of a crosstalk signal can be represented as given in Equation 1.
                              f          e                =                              f                          C              L                                +                                                    f                                  b                  L                                                                              f                                      b                    L                                                  +                                  f                                      b                    H                                                                        ⁢                          (                                                f                                      C                    H                                                  -                                  f                                      C                    L                                                              )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
(where fCL denotes a carrier frequency of a low-band signal in the DSL, fCH denotes a carrier frequency of a high-band signal in the DSL, fbL denotes a symbol rate of the low-band signal, and fbH denotes a symbol rate of the high-band signal).
The bandwidth overlapping together with the use of imperfect wire shielding results in a crosstalk signal in the received signal. In FIG. 1, the region A shows the spectrum of the crosstalk signal formed by the partially overlapped received signals in the frequency domain.
Assume that the Tx signal is pulse-shaped by using a square-root raised cosine filter (SRCF) with a roll-off factor of α. Then, the spectrum of the crosstalk signal is located in the frequency range of Equation 2.
                                          f                          C              H                                -                                                    f                                  b                  H                                            2                        ⁢                          (                              1                +                α                            )                                      ≤        f        ≤                              f                          C              L                                +                                                    f                                  b                  L                                            2                        ⁢                          (                              1                +                α                            )                                                          [                  Equation          ⁢                                          ⁢          2                ]            
In order to measure the amount of the spectral overlapping, a carrier spacing ratio must be defined as given in Equation 3. The spacing ratio ζ represents frequency spacing between the center frequencies of the Tx and Rx signals normalized by the bandwidth of the Tx signal.
                    ζ        ≡                              2            ⁢                          (                                                f                                      C                    H                                                  -                                  f                                      C                    L                                                              )                                                          (                                                f                                      b                    L                                                  +                                  f                                      b                    H                                                              )                        ⁢                          (                              1                +                α                            )                                                          [                  Equation          ⁢                                          ⁢          3                ]            
When the spacing ratio is less than 1 (i.e., ζ<1.0), it implies that the two signals are overlapped in the frequency domain.
The Nyquist rate IDXC is used for crosstalk signal cancellation in data communications, and an interpolation ratio L of a polyphase structure is defined as given in Equation 4.
                    L        ≥                  {                                                                                                                ⌈                                                                                                    2                            ⁢                                                                                                                  ⁢                                                          f                                                              C                                H                                                                                                              +                                                                                    f                                                              b                                H                                                                                      ⁡                                                          (                                                              1                                +                                α                                                            )                                                                                                                                f                                                      b                            L                                                                                              ⌉                                        ;                                                                                        in                    ⁢                                                                                  ⁢                    the                    ⁢                                                                                  ⁢                    premises                                                                                                                                          ⌈                                                                                                    2                            ⁢                                                                                                                  ⁢                                                          f                                                              C                                H                                                                                                              +                                                                                    f                                                              b                                H                                                                                      ⁡                                                          (                                                              1                                +                                α                                                            )                                                                                                                                f                                                      b                            H                                                                                              ⌉                                        ;                                                                                        in                    ⁢                                                                                  ⁢                    the                    ⁢                                                                                  ⁢                    central                    ⁢                                                                                  ⁢                    office                                                                        .                                              [                  Equation          ⁢                                          ⁢          4                ]            
Where ┌x┐ denotes the largest integer less than or equal to x.
When the crosstalk signal has a duration of N symbols and a conventional least mean square (LMS) adaptation algorithm is employed, each L-polyphase sub-canceller requires 4N MACs for filtering and adaptation of N complex-valued coefficients for each symbol duration. Herein, the MAC is a unit for one multiplication and accumulation.
Therefore, the L-polyphase sub-canceller requires a computational complexity of 8LNfbL MACs per second. As an example, assume that a symmetric DSL (SDSL) with a symbol rate of 256 Kbauds and ζ=0.83 is implemented. In this assumption, a conventional Nyquist rate IDXC requires a computational complexity of more than 640×106 MACs for a crosstalk signal with a span of 80 μs microseconds. However, it is difficult to implement such a sub-canceller that performs the computation by using a conventional digital signal processor.